Koji rice

It is said that the most important aspect of making sake is the koji. If you can’t make good koji, you can’t make good sake. What makes good koji you ask? In general, good koji rice has all grains completely covered in koji, and the koji hyphae penetrate deep into the grain (it is the enzymes secreted by the hyphal tips (the ends of the mold cells) that lead to saccharification). The whole process takes about 48 hours, and for the final half of the time, the koji rice is cultivated in special cedar “boxes” seen in the above photo. In the box, the koji rice is about 8-10 cm in thickness, and is covered with several blankets to keep in the heat and moisture. The temperature is very important in the process and will dictate the quality of the koji. The koji spores are sprinkled on the rice when the rice temp is around 32-33C, and throughout the process, the temp gradually rises until approx 42-43C when the growth is complete. It is somewhat difficult to see in the above photo, but there is a remote thermometer monitoring one of the boxes- as the temp rises, the koji rice is gradually thinned by moving a board further along the slots that are visible inside the box.

One of my favorite tasks is the process called ‘de-koji’ (出麹), which literally translates to ‘leaving koji’, as this is the time when the koji is taken out of the boxes, moved to a different room with an air temp of around 12C (as opposed to 32C in the koji room), and spread out on a large sheet. One reason I like dekoji is because now in Nagano it is freezing cold (today was -12C), and entering into a 32C room is feels like a mini tropical vacation. The other reason is that this is the time when the koji can be evaluated- does it taste sweet? does it have the proper smell (good koji gives off a chestnut smell)? are all the grains covered? not too dry? Good koji also makes a nice snack.

Well, that is a little bit of info about koji. thanks for reading.

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4 Responses to “Koji rice”

“It is said that the most important aspect of making sake is the koji.” I have seen this said many times and I am sure its true. However, I have never seen anything specific about how the koji can impact the sake. How would the sake change if the rice was not well covered by the white fuze?

John Gauntner has referred to a time when he could tasted the cedar in the sake because the cedar in the koji room was too young. This was presumable good koji.

There are a number of different types of koji-kin that can be used but I have never seen anything about why use one over the other (or what the one or other is). What might the difference be for a koji best suited for Miso verses one best suited for sake. Here I am only talking about yellow koji.

Hi Will,
Although ‘good’ koji is well-covered by white fuzz (the fungal hyphae), this alone does not make the koji good. The hyphae must penetrate the rice grains, and then secrete enzymes to turn the sugar to starch. I heard from a past toji at the brewery that viewing several grains of finished koji on a small flashlight (ie, vertical) will give you a good idea of the degree of inward growth. That said, for top-quality daiginjo, rice not completely covered in koji is ideal.
As far how it impacts the sake, it is said that without strong koji (ie good growth), the sake will not have umami (a vague term for ‘deliciousness’). I imagine that apart from supplying the yeast with the necessary sugar, strong koji also secretes other products capable of transforming the starch and other molecules into things we can taste. If the koji is weak, they are either in too few numbers, or not in the proper to condition to secrete these enzymes, or both.
As for yellow koji (Aspergillus oryzae), there are many strains and I do not have a good handle of what makes a good Miso koji-kin. For sake koji, high levels of alpha-amylase and glucoamylase are desired. Let me do some more research!